On a recordable optical disc such as a minidisc or a CD-R that is rotated at a constant linear velocity (CLV), address information indicating absolute positions on the optical disc and rotation control information for the optical disc are recorded beforehand by using wobbling guide grooves. The address information is encoded using recording codes (for example, biphase codes) that enable easy clock synchronization during reproduction, and signals obtained by FM-modulating these recording codes are recorded on the optical disc in such a way as to wobble through the guide grooves.
Such an optical disc is reproduced by obtaining from the optical disc FM-modulated signals formed by means of wobbling through the guide grooves, and then FM-demodulating the signals to extract biphase-encoded address information. Furthermore, a PLL (Phase Locked Loop) circuit is used to extract synchronizing clocks, which are then biphase-decoded to detect addresses. The rotation control information for the optical disc is synchronizing clocks extracted from the biphase signals and controls a spindle motor to rotate the optical disc so that the frequency and phase of the synchronizing clocks have predetermined values.
In such a system, efforts are being made to reduce the track pitch in order to accommodate the increased density of information recorded on an optical disc, but simply reducing the track pitch may cause crosstalk from adjacent wobbling grooves. An optical disc 5 of a track format such as that shown in FIG. 4 is used to avoid interference by crosstalk. The optical disc 5 has two types of land areas 3, 4 formed in a double spiral structure. The land areas 3 and 4 are two discontinuous areas. The land area 3 is sandwiched between a non-wobbling groove 2, which is shown to the left of the land area 3, and a wobbling groove 1, which is shown to the right of the land area 3, and information is recorded on this area 3. The land area 4 is sandwiched between the wobbling groove 1 shown to the left of the land area 4 and the non-wobbling groove 2 shown to the right of the land area 4, and as in the land area 3, information is recorded on this area 4. The wobbling groove 1 is wobbled to provide predetermined signals including address information and rotation control information (hereafter referred to as "wobble information"), while the non-wobbling groove 2 is not wobbled. The interval T between the wobbling grooves 1 is twice the track pitch Tp between the wobbling groove 1 and the non-wobbling groove 2 so that the structure is unlikely to be affected by crosstalk.
In the optical disc 5 of such a track format, the method called "differential push-pull" is used to control the tracking of a beam light impinging on one of the land areas 3 and 4. In this differential push-pull, the land area 3 is irradiated with a main beam M, and the wobbling groove 1 and the non-wobbling groove 2 are irradiated with subbeams SB1, SB2, respectively. Reflected lights of the subbeams are received by split photo detectors 21, 23, as shown in FIG. 5. Detection outputs from the split photo detectors 21, 23 are inputted to differential amplifiers 24, 25, respectively, to obtain difference signals (push-pull signals PP1, PP2). The push-pull signals PP1, PP2 are demodulated so that wobble information is obtained using one of the push-pull signals PP1, PP2 that detects a reflected light from the wobbling groove 1. Thus, the wobble information can be detected from either the subbeam SB1 or SB2. For example, Japanese Patent No. 1917370 describes tracking control in which, of three difference signals obtained by means of irradiation with the main beam MB and the two subbeams SB1, SB2, any two difference signals are calculated to determine their difference in order to obtain a tracking error signal.
Due to their double spiral structure shown in FIG. 4, the two land areas 3, 4 on which information is recorded are arranged to sandwich the wobbling groove 1. Since wobble information is recorded in the wobbling groove 1, address (absolute position) information on the two land areas 3, 4 is obtained from the shared wobbling groove 1 sandwiched by the two land areas 3, 4. Thus, wobble information is obtained from the wobbling groove 1 whether the main beam MB is impinging on the land area 3 or 4.
Specifically, if the main beam MB is impinging on the land area 3, wobble information is obtained by detecting a wobble signal from a reflected light of the subbeam SB2, whereas if the main beam MB is impinging on the land area 4 as shown by the dotted line, wobble information is obtained by detecting a wobble signal from a reflected light of the subbeam SB1. Thus, by determining which of the subbeams SB1, SB2 provides wobble information, it can be determined which of the two land areas 3, 4 is irradiated with the main beam MB.
This method, however, requires the two push-pull signals PP1, PP2 obtained from reflected lights of the two subbeams SB1, SB2 to be constantly demodulated by respective demodulating means. That is, two FM demodulating circuits 29 and two biphase demodulators 30 must be provided and constantly operated. Furthermore, FM demodulation and biphase decoding involve processing delay time, so it cannot be determined whether the main beam MB is impinging on the land area 3 or 4, immediately after the beam has focused on the land area.
An object of this invention is to provide a method of processing signals for an optical disc device and an optical disc device that can immediately determine the on-track state of the main beam light using a simple configuration.